1. Field of the Invention
The present invention relates to power steering systems.
2. Description of the Prior Art
There are many previously known power steering systems. Virtually all power steering systems include a hydraulic cylinder having a hydraulic piston slidably disposed within the cylinder. The piston is mechanically connected to the steering mechanism of the vehicle so that displacement of the piston within the cylinder simultaneously turns the vehicle's dirigible wheels.
In order to control actuation of the piston within the hydraulic cylinder, typically a valve assembly is provided with the steering system. The valve assembly includes compliantly coupled rotary input and valve sleeve members which are rotatably mounted within a valve housing. The rotary input and valve sleeve members are typically connected to the vehicle's steering wheel and dirigible wheels, respectively. Thus, as torque is applied to the steering wheel, rotational displacement of the rotary input member relative to the valve sleeve occurs and flow control orifices within the valve assembly are selectively opened or closed in order to effect a differential pressure across and/or fluid flow through the power cylinder and, in doing so, enable the power steering function in a well known fashion.
Valve assemblies utilizing an open center valve are most commonly employed in power steering systems. In an open center valve, all flow control orifices have a normally open configuration whereby fluid flow through the valve is allowed even when the rotary input member of the valve is at an "at rest" position. Conversely, when torque is applied to the rotary input member, selected ones of the flow control orifices progressively close. In combination with continuous fluid flow through the valve, this results in increasing system pressure which is substantially coupled to the appropriate side of the hydraulic piston.
Although rarely used, valve assemblies utilizing a closed center valve have also been employed in power steering systems. In such prior art closed center valves all flow control orifices are closed with the rotary input member at an "at rest" position. Thus, in the "at rest" position fluid flow through the valve is substantially precluded. When torque is applied to the rotary input member selected ones of the flow control orifices progressively open and, so long as a sufficiently pressurized source of fluid is present, move the piston at a velocity nominally proportional to the area of the open flow control orifices. Such systems typically utilize an accumulator charged by an intermittently operating pumping subsystem as the pressurized source of fluid.
In order to control actuation of the hydraulic piston within the hydraulic cylinder, valve assemblies typically include four sets of valve orifices (hereinafter called valves) namely two input valves and two output valves, each of which has an inlet and outlet. Typically the inlet of the input valves are fluidly connected to the pump outlet while the outlets from the input valves are fluidly connected to the inlets of the output valves. The outlets from the outlet valves are, in turn, fluidly connected to the inlet of the pump.
One chamber of the power cylinder is then fluidly connected to the junction between one input and one output valve while, similarly, the second fluid chamber in the power cylinder is fluidly connected to the junction of the other input and output valve. Consequently, by proper design of the valve assembly for the power steering system, the input and output valves, are selectively closed or opened progressively in response to a torque applied to the rotary input member by the steering wheel in order to create a differential pressure across and/or fluid flow through the power cylinder chambers. It is the differential pressure which drives the piston and, in turn, provides the desired power assist for steering the vehicle.
The selection of closed center valves or open center valves both have their own advantages and disadvantages. For example, open center valves in the valve assembly provide relatively good control of the steering mechanism, but require constant fluid flow through the valve even when the valve is at an "at rest" position. As such, the pump for a system using open center valves consumes relatively large amounts of power.
Closed center valves, on the other hand, can be much more power efficient than open center valves since the closed center valve prohibits fluid flow (except for minor leakage) when the valve assembly is at an "at rest" position. Such increased efficiency can be realized, even in prior art systems which consume fully pressurized fluid for every steering wheel motion, because total fluid volumes consumed are far less than with valve assemblies using open center valves. However, such prior art systems have not been generally accepted because of undesirable steering feel and safety concerns relating to accumulator storage of relatively large volumes of hydraulic fluid under pressure.